Quantifying plastic contributions to different components of the river channel and floodplain

Rivers are one of the main conduits that deliver plastic from land into the sea, and also act as reservoirs for plastic retention. Yet, our understanding of the extent of river exposure to plastic pollution remains limited. In particular, there has been no comprehensive quantification of the contributions from different river compartments, such as the surface, water column, riverbank and floodplain, to the overall river plastic transport and storage. Here, we provide an initial quantification of these contributions. First, we identified the main relevant transport processes for each river compartment considered. We then estimated the transport and storage terms, by harmonizing available observations on surface, suspended and floodplain plastic. This approach was applied to two river sections in the Netherlands, with a focus on macroplastics (≥ 2.5 cm). Our analysis revealed that for the studied river sections, suspended plastics account for over 96% of items transported within the river channel, while their relative contribution to mass transport was only 30-37% (depending on the river section considered). Surface plastics predominantly consisted of heavier items (mean mass: 7.1 g/#), whereas suspended plastics were dominated by lighter fragments (mean mass: 0.1 g/#). Additionally, the majority (98%) of plastic mass was stored within the floodplains, with the river channel accounting for only 2% of the total storage. Our study developed, and demonstrates, a harmonised approach for quantifying plastic transport and storage across different river compartments, providing a replicable methodology  which will be applicable to many different river environments. Our findings emphasize the importance of adopting a systematic monitoring approach, across the range of river compartments encountered, in order to achieve a coherent and  comprehensive understanding of riverine plastic pollution dynamics.